Rayon Carrie Weaver & Rachael Blondy


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Rayon is made from wood-pulp, which is a naturally occurring, cellulose-based raw material.
Viscose is the most common cellulosic fiber, and is simply a cellulosic fiber obtained through the viscose process. In the United States, these viscose fibers are known as rayon.
Viscose rayon fibers are biodegradable.




In 1664, an English naturalist Robert Hooke found out that some artificial filaments can be spun from something similar to what silkworms secrete, making an "artificial silk". In 1855, Frenchman George Audemars found that by dipping a needle into a viscous solution of mulberry pulp bark and gummy rubber, he could make a thread. This is how the first cellulose-based rayon samples were made.
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Types of Rayon:

  • Regular rayon
  • High wet modulus rayon (HWM rayon)- a modified viscose rayon and has much better resistance to washing
  • Cupramonium rayon- made by converting cellulose into a soluble compound (combined with copper and ammonia); it is usually used to make lightweight summer clothing or combined with cotton
  • Saponified rayon- cellulose acetate reconverted to cellulose
  • High tenacity rayon- similar to regular rayon but much stronger
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Rayon Fiber Characteristics:

  • Highly absorbent
  • Soft and comfortable
  • Easy to dye
  • Drapes well
  • Does not build up static electricity
  • Moderate resistance to alkalis and acids- can be damage easily by even relatively weak acids
  • High flammability (but there are certain types made to be flame retardant called visil rayon, which have silica built into the fibers during manufacturing)
Properties are similar to cotton and other cellulosic fibers.

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Thermal properties:

  • Viscose rayon loses strength above 149° C
  • Chars and decomposes at 177 to 204° C.
  • It does not melt or stick at elevated temperatures.

Chemical properties:
  • Hot dilute acids attack rayon, whereas bases do not seem to significantly attack rayon.
  • Attacked by bleaches at very high concentrations and by mildew under severe hot and moist conditions.
  • Prolonged exposure to sunlight causes loss of strength because of degradation of cellulose chains.

Abrasion resistance is fair and rayon resists pill formation. Rayon has both poor crease recovery and crease retention.


Rayon can imitate the texture and feel of cotton, silk, wool, and linen. It is not very durable when wet and has very low elastic recovery, but HWM rayon is much more durable. (washing for rayon is recommended dry-clean only, but HWM is machine washable).


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Rayon Fiber


Major uses of Rayon:
Accessories, blouses, dresses, jackets, lingerie, linings, hats, slacks, sports shirts, sportswear, suits, ties, work clothes, bedspreads, blankets, curtains, draperies, sheets, slipcovers, tablecloths, upholstery, industrial products, medical surgical products, nonwoven products, tire cord, feminine hygiene products, pipe cleaners


Cross-linkage does occur with rayon because cross-linkage with monovinyl polymer fibers allows for larger surface area, higher reaction or adsorption velocity, higher reaction efficiency, liquid permeability, and various other uses based on the form of fibers. Monovinyl polymers can't have cross-linkage and be spun naturally, so steps are taken involving blending with other polymers for cross-linkage to occur, to eventually create the rayon fibers.





To make viscose fibers from cellulose (from wood pulp):
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(This is one of 6 different processes.)

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Cupramonium and saponified cellulose acetate process:


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Manufacturing viscose rayon involves the following process:

"(1) Steeping, (2) Pressing, (3) Shredding, (4) Aging, (5) Xanthation, (6) Dissolving, (7)Ripening, (8) Filtering, (9) Degassing, (10) Spinning, (11) Drawing, (12)Washing, (13) Cutting.
Steeping: Cellulose pulp is immersed in 17-20% aqueous sodium hydroxide (NaOH) at a temperature in the range of 18 to 25° C in order to swell the cellulose fibers and to convert cellulose to alkali cellulose.
(C6H10O5)n + nNaOH ---> (C6H9O4ONa)n + nH2O

Pressing: The swollen alkali cellulose mass is pressed to a wet weight equivalent of 2.5 to 3.0 times the original pulp weight to obtain an accurate ratio of alkali to cellulose.

Shredding: The pressed alkali cellulose is shredded mechanically to yield finely divided, fluffy particles called "crumbs". This step provides increased surface area of the alkali cellulose, thereby increasing its ability to react in the steps that follow.

Aging: The alkali cellulose is aged under controlled conditions of time and temperature (between 18 and 30° C) in order to depolymerize the cellulose to the desired degree of polymerization. In this step the average molecular weight of the original pulp is reduced by a factor of two to three. Reduction of the cellulose is done to get a viscose solution of right viscosity and cellulose concentration.

Xanthation: In this step the aged alkali cellulose crumbs are placed in vats and are allowed to react with carbon disulphide under controlled temperature (20 to 30° C) to form cellulose xanthate.
(C6H9O4ONa)n + nCS2 ----> (C6H9O4O-SC-SNa)n

Side reactions that occur along with the conversion of alkali cellulose to cellulose xanthate are responsible for the orange color of the xanthate crumb and also the resulting viscose solution.
The orange cellulose xanthate crumb is dissolved in dilute sodium hydroxide at 15 to 20° C under high-shear mixing conditions to obtain a viscous orange colored solution called "viscose", which is the basis for the manufacturing process. The viscose solution is then filtered (to get out the insoluble fiber material) and is deaerated.

(6) Dissolving: The yellow crumb is dissolved in aqueous caustic solution. The large xanthate substituents on the cellulose force the chains apart, reducing the interchain hydrogen bonds and allowing water molecules to solvate and separate the chains, leading to solution of the otherwise insoluble cellulose. Because of the blocks of un-xanthated cellulose in the crystalline regions, the yellow crumb is not completely soluble at this stage. Because the cellulose xanthate solution (or more accurately, suspension) has a very high viscosity, it has been termed "viscose"[13].

(7) Ripening: The viscose is allowed to stand for a period of time to "ripen". Two important process occur during ripening: Redistribution and loss of xanthate groups. The reversible xanthation reaction allows some of the xanthate groups to revert to cellulosic hydroxyls and free CS2. This free CS2 can then escape or react with other hydroxyl on other portions of the cellulose chain. In this way, the ordered, or crystalline, regions are gradually broken down and more complete solution is achieved. The CS2 that is lost reduces the solubility of the cellulose and facilitates regeneration of the cellulose after it is formed into a filament.
(C6H9O4O-SC-SNa)n + nH2O ---> (C6H10O5)n + nCS2 + nNaOH

(8) Filtering: The viscose is filtered to remove undissolved materials that might disrupt the spinning process or cause defects in the rayon filament[13].

(9) Degassing: Bubbles of air entrapped in the viscose must be removed prior to extrusion or they would cause voids, or weak spots, in the fine rayon filaments[13].

(10) Spinning - (Wet Spinning): Production of Viscose Rayon Filament: The viscose solution is metered through a spinnerette into a spin bath containing sulphuric acid (necessary to acidify the sodium cellulose xanthate), sodium sulphate (necessary to impart a high salt content to the bath which is useful in rapid coagulation of viscose), and zinc sulphate (exchange with sodium xanthate to form zinc xanthate, to cross-link the cellulose molecules). Once the cellulose xanthate is neutralized and acidified, rapid coagulation of the rayon filaments occurs which is followed by simultaneous stretching and decomposition of cellulose xanthate to regenerated cellulose. Stretching and decomposition are vital for getting the desired tenacity and other properties of rayon. Slow regeneration of cellulose and stretching of rayon will lead to greater areas of crystallinity within the fiber, as is done with high-tenacity rayons.

The dilute sulphuric acid decomposes the xanthate and regenerates cellulose by the process of wet spinning. The outer portion of the xanthate is decomposed in the acid bath, forming a cellulose skin on the fiber. Sodium and zinc sulphates control the rate of decomposition (of cellulose xanthate to cellulose) and fiber formation.
(C6H9O4O-SC-SNa)n + (n/2)H2SO4 --> (C6H10O5)n + nCS2 + (n/2)Na2SO4

Elongation-at-break is seen to decrease with an increase in the degree of crystallinity and orientation of rayon.

(11) Drawing: The rayon filaments are stretched while the cellulose chains are still relatively mobile. This causes the chains to stretch out and orient along the fiber axis. As the chains become more parallel, interchain hydrogen bonds form, giving the filaments the properties necessary for use as textile fibers[13].

(12) Washing: The freshly regenerated rayon contains many salts and other water soluble impurities which need to be removed. Several different washing techniques may be used[13].

(13) Cutting: If the rayon is to be used as staple (i.e., discreet lengths of fiber), the group of filaments (termed "tow") is passed through a rotary cutter to provide a fiber which can be processed in much the same way as cotton[13]."
[ http://www.mindfully.org/Plastic/Cellulose/Rayon-Fiber.htm]





Citations:

"A-level Applied Science/Colour Chemistry/Fibres/Cellulose". Wikibooks. 15 May 2008 http://en.wikibooks.org/wiki/A-level_Applied_Science/Colour_Chemistry/Fibres/Cellulose.

Jangala, Praveen Kumar and Haoming Rong. "Making Rayon Fiber 1999". University of TN Textile Science. 15 May 2008 <http://www.mindfully.org/Plastic/Cellulose/Rayon-Fiber.htm>.

"Method for Producing Porous Fibers". FreePatentsOnline. 13 May 2008 http://www.freepatentsonline.com/4107384.html.

"Pipe Cleaners". Habana Premium Cigar Shoppe. 13 May 2008 <http://www.pipesandcigars.com/pipecleaners.html>.

"Rayon". Wikipedia. 12 May 2008 <http://en.wikipedia.org/wiki/Rayon>.

"Rayon Fiber". Fibersource. 12 May 2008 <http://www.fibersource.com/f-tutor/rayon.htm>.

"Rayon Viscose". Swicofil. 13 May 2008 <http://www.swicofil.com/viscose.html>.

Smith, Joyce A.. "Rayon- The Multi-Faceted Fiber". Ohio State University. 13 May 2008 <http://ohioline.osu.edu/hyg-fact/5000/5538.html>.

"Viscose". CIRFS- International Rayon and Synthetic Fibres Committee. 13 May 2008 <http://www.cirfs.org/03_02.htm>.